double * lsx_design_lpf(
double Fp, /* End of pass-band */
double Fs, /* Start of stop-band */
double Fn, /* Nyquist freq; e.g. 0.5, 1, PI */
double att, /* Stop-band attenuation in dB */
int * num_taps, /* 0: value will be estimated */
int k, /* >0: number of phases; <0: num_taps �ß 1 (mod -k) */
double beta) /* <0: value will be estimated */
{
int n = *num_taps, phases = max(k, 1), modulo = max(-k, 1);
double tr_bw, Fc, rho = phases == 1? .5 : att < 120? .63 : .75;
Fp /= fabs(Fn), Fs /= fabs(Fn); /* Normalise to Fn = 1 */
tr_bw = .5 * (Fs - Fp); /* Transition band-width: 6dB to stop points */
tr_bw /= phases, Fs /= phases;
tr_bw = min(tr_bw, .5 * Fs);
Fc = Fs - tr_bw;
assert(Fc - tr_bw >= 0);
lsx_kaiser_params(att, Fc, tr_bw, &beta, num_taps);
if (!n)
*num_taps = phases > 1 ? *num_taps / phases * phases + phases - 1
: (*num_taps + modulo - 2) / modulo * modulo + 1;
return Fn < 0? 0 : lsx_make_lpf(
*num_taps, Fc, beta, rho, (double)phases, sox_false);
}
static double * lpf(double Fn, double Fc, double tbw, int * num_taps, double att, double * beta, sox_bool round)
{
if ((Fc /= Fn) <= 0 || Fc >= 1) {
*num_taps = 0;
return NULL;
}
att = att? att : 120;
*beta = *beta < 0? lsx_kaiser_beta(att) : *beta;
if (!*num_taps) {
int n = lsx_lpf_num_taps(att, (tbw? tbw / Fn : .05) * .5, 0);
*num_taps = range_limit(n, 11, 32767);
if (round)
*num_taps = 1 + 2 * (int)((int)((*num_taps / 2) * Fc + .5) / Fc + .5);
lsx_report("num taps = %i (from %i)", *num_taps, n);
}
return lsx_make_lpf(*num_taps |= 1, Fc, *beta, 1., sox_false);
}
